WASHINGTON -- As the Senate opened debate on campaign-finance overhaul, Wisconsin Sen. Russell Feingold warned fellow Democrats against backing away from their past support for his legislation to ban large "soft money" donations from federal elections.

"Second thoughts are fine. It's how you vote," Mr. Feingold said bluntly, and the tough tone was striking from a man who was been overshadowed by his flamboyant partner in reform, Arizona Republican Sen. John McCain. In an early test of strength Monday...